1. Field of the Invention
The invention relates to a hot-moldable laminated sheet which is composed of a core and parts hot-pressed with said core and has a decorative layer on one or both surfaces of the core and of the parts bonded thereto, and a process for the production of a hot-moldable laminated sheet.
2. Description of the Related Art
For the interior finish in kitchens and bathrooms, the cladding of facades of buildings and the furnishing of chemical and physical laboratories, sheets which have rounded edges and a curved shape are produced. These sheets frequently have a decorative surface, and the starting materials are support layers which are adhesively bonded to one another.
Thus, British Patent 1 600 469 (corresponds to German Patent 2 823 669) discloses a process in which the sheet is heated to such an extent that the support layers or the groups of layers which consist of a plurality of support layers bonded to one another, which layers or groups are bonded to one another by means of a thermoplastic bonder, are displaced relative to one another in the direction of the sheet surface during molding in the heated molding region, and that in each case separation of the binder joint is prevented by mold retaining elements of the bending means during or after the molding or only after the molding, at least until the binder has solidified again as a result of cooling.
This gives a laminated synthetic resin sheet which comprises a plurality of support layers hot-pressed with one another and impregnated with thermosetting binders and can be molded at a molding temperature which is higher than the hot-pressing temperature. Between the support layers or between groups of layers consisting in each case of a plurality of support layers bonded to one another are layers of a thermoplastic binder whose melting point is the same as or lower than the molding temperature.
U.S. Pat. No. 4,503,115 (corresponds to European Patent 0 081 147) describes a decorative structural sheet which is suitable for outdoor applications and comprises a pressed core of fibers which are surrounded by heat-cured phenol/formaldehyde resin and has a decorative layer on one or both surfaces of the core. The core comprises wood fibers and/or cellulose fibers having a maximum length of 20 mm, which are coated with a heat-curable phenol/formaldehyde resin in aqueous solution or dispersion. One embodiment of this structural sheet is produced by a procedure in which one or more mechanically precompacted layers arranged one on top of the other and based on wood particles which are impregnated with the heat-curable phenol/formaldehyde resin are hot-pressed with formation of the core of the structural sheet, with the result that the synthetic resin is cured. The surfaces of the core are provided with one or two decorative layers. The wood fibers impregnated with the resin solution or resin dispersion are dried to a residual moisture content of from 2 to 15% by weight, the resin partially curing. The dried fibers are entangled to form a web-like mat, which is mechanically precompacted and then heated and pressed in one or more layers arranged one on top of the other, under resin-curing conditions. The decorative layers are applied to the precompacted mat or, after pressing, to the core layer.
Such hot-molded decorative high-pressure laminated sheets (HPL) are so-called compact moldings whose structure either corresponds to that of compact sheets according to EN 438-1 or DIN 16926 or ISO 4586 or is similar to such a structure. Components or layers of such compact sheets consist of the same material and are produced in the same high-pressure presses as other sheets based on curable resin and, depending on the molding process, may from case to case contain adhesive layers. Compact moldings are self-supporting elements having stable rounded edges and, like compact sheets, have a decorative coloring on one or both sides, with a smooth or structured surface, and generally have closed cut edges.
Processes in which melamine resin-impregnated decorative webs and phenol resin-impregnated core paper webs in S-shaped or L-shaped structures of defined dimensions are cured under heat and high pressure are also known. In another known process, the thickness of the compact sheets is limited on one side in regions where bending is to be performed, by inserting separating strips during the production, the thickness depending on the desired bending radius. In the thinner regions of the sheet, subsequent shaping in stationary bending units under heat is then possible. After cooling in a clamping apparatus, the parts remain shaped, the mechanical strength of the round region being determined by the remaining wall thickness.
In a further known process, the compact sheet is milled on one side in the regions to be molded, down to a small wall thickness which is dependent on the desired bending radius, and is shaped in stationary bending units under heat. The milled-out cavities are filled with curable synthetic resins or strengthened by inserting fitting pieces in the clamping apparatus.
The production of moldings from compact sheets and a subsequently bonded HPL compact sheet is effected in a further known process by milling the compact sheet prior to bonding in the area subsequently to be molded or filling said area with a spacer strip. Another possibility comprises joining two compact sheets as a support at the intended angle and milling one of the edges to form a convex rounded edge. The HPL compact sheet is then adhesively bonded to this milled surface. Concave rounded edges can be produced by milling the back and removing the spacer strip.
In the production of compact moldings from single HPL layers, these single layers in thicknesses of up to 1 mm are laminated with core sheets ground on both sides, to give sheet packets of the desired thickness, and then bonded together in clamping molds with, for example, solvent-free, two-component adhesives. The adhesive bonding of the nonabsorptive single HPL layers to give tight joints sets high requirements with respect to the two-component adhesives. The laminated sheet packets are then bent in the desired manner in a bending apparatus.
The known processes in which the material is milled in the molding region and the cavities in the covering are filled with synthetic resin casting compound or strengthened by inserting fitting pieces in the clamping apparatus are expensive owing to the large number of processing steps, such as milling, bending, filling or production of fitting pieces and reworking, and the product quality is unsatisfactory since, for example, cylindrical shapes are often irregular, hairline cracks often occur at the ends of elements and the material can only be used as an element which is decorative on one side. In the case of molding in a molding press, the product quality is good but the dies for the molding are very expensive and, owing their poor flexibility, cannot be used for different shapes.
The moldability of uncured layers of HPL compact sheets which have been laminated together to give a sheet, the outside of such a sheet being provided with melamine or acrylic resin top layers, is limited by the small extensibility of the decorative top layers.
It is the object of the invention further to develop a sheet of the type described at the outset in such a way that, without performing molding work, a sheet which is thicker and rounded in the edge regions and has constant density and a decorative layer or layers substantially free of hairline cracks, fissures and similar defects is obtained.
This object is achieved, according to the invention, if a prefabricated flat sheet of wood and/or cellulose fibers, which have been impregnated with heat-cured phenol/formaldehyde resin, is provided as the core and if the sheet is thickened along one or more of its edge regions with the material of which the prefabricated sheet consists.
The prefabricated sheet is disclosed, for example, in the abovementioned U.S. Pat. No. 4,503,115 (corresponds to European Patent 0 081 147) and, depending on the desired thickness, consists of one or more flat, rectangular sheets which are composed of wood and/or cellulose fibers which have been impregnated with heat-curable phenol/formaldehyde resin. A plurality of mechanically precompacted layers arranged one on top of the other are pressed together with application of heat. The flat sheet prefabricated in this manner is also referred to as a prepreg. In a further development of the invention, the material comprises prefabricated strips which have a rectangular cross-section, a rectangle-like cross-section having an arc-shaped cross-sectional side or a trapezoidal cross-section having an oblique side. Expediently, the strips terminate at edges of the prefabricated flat sheet, and the arc-shaped cross-sectional side of the strips or the oblique side of the strips is directed away from the edges toward the interior of the sheet. It is also possible for the arc-shaped cross-sectional side or the oblique side to be directed toward the edge of the sheet.
In a further embodiment of the laminated sheet, the material for thickening the edge regions comprises loose wood or cellulose fibers which are impregnated with resin and fill one or more cavities of a mold between which and a further mold the prefabricated flat sheet and the material together with one or two decorative layers is pressed to form the laminated sheet.
Within the scope of the invention, it is also intended to provide a process for the production of laminated sheets which manages with a very small number of process steps. This object is achieved, according to the process, by inserting a prefabricated sheet of wood and/or cellulose fibers into an opened press which contains a mold equipped with one or more cavities and a smooth mold, arranging the material of which the sheet is made along at least one of the edge regions of the sheet, applying one or two decorative layers to the surface(s) of the sheet combined with the material and hot-pressing all parts present between the molds.
In one embodiment of the process, at least one strip produced from the same material as the sheet is applied as material along one of the edge regions of the sheet.
In another embodiment of the process, one or more cavities of the mold is or are filled with the wood and/or cellulose fiber material of which the prefabricated sheet consists and one or two decorative layers are applied to the surface(s) of the sheet combined with the wood and/or cellulose fiber material in the cavity or cavities and pressed together with heat and pressure between the molds. According to the process, the parts are pressed together between the molds at a temperature of 130 to 180xc2x0 C. and at a pressure of 60 to 110 bar.
The laminated sheet has the advantages that it has a uniform density and substantially homogeneous physical, chemical and technical properties and that, in the edge regions, the thickness and the shape of the thickened edge regions can be varied in a simple manner. Another advantage is that the laminated sheet can be produced using any conventional press, it being necessary in each case to replace only a single mold in order to obtain another embodiment of the laminated sheet.
Further objects, features and advantages of the present invention will become apparent from the detailed description that follows.